286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



certainly not in existence when the radio waves we receive now started 

 on their journey. It is fortunate that the object was not at any greater 

 distance for it would then have been beyond the limits of the visible 

 universe as seen with the Palomar telescope. By careful photography 

 the telescope shows that a remarkable catastrophe is taking place out 

 there. Two galaxies, two huge systems of stars and gas, are involved 

 in a collision. Plate 2, figure 1, shows the galaxies in contact, but it 

 is difficult to imagine that the spots and surrounding halo actually 

 represent a cloud of stars some 3 X 10 17 km. across. Collisions of 

 this kind are extremely rare and we would probably have to see well 

 beyond our present range before we found another face-to-face con- 

 tact like that in Cygnus A. The consequences of galactic collisions 

 have already been studied. Remarkably enough, the stars in the 

 system are hardly affected at all; interstellar distances are so great 

 that the star systems can pass through each other with only minor 

 perturbations. The gas between the stars, however, meets with great 

 violence. Part of the kinetic energy of the collision is emitted as 

 radio waves ; indeed, the process is extremely efficient, about 5 percent 

 of the energy being converted in this way. It seems that collision and 

 violent motion in gas clouds are an essential requirement for the for- 

 mation of a radio source. Cassiopeia A contains gaseous filaments 

 in rapid motion, Cygnus A is formed by gas clouds in collision, and 

 we shall see that other radio stars are associated with this condition. 

 It has probably taken a million years or so for the galaxies to pass 

 through each other. Bearing in mind the fact that light takes 200 

 million years for the journey, we realize that the actual collision 

 process must have been completed long ago and there will now be 

 left two remarkable galaxies in space cleared of dust and gas, while 

 between them will be a hot gaseous nebula, far larger than any that 

 we encounter in the local galaxy. But these objects will not be visible 

 to astronomers until a million years have passed. 



There is one radio star that was observed in A. D. 1054, 12 years be- 

 fore William the Conqueror landed in England. In this year a star 

 in the constellation of Taurus, the Bull, exploded, leaving an object 

 which we now call the Crab nebula. The sudden increase in bright- 

 ness was seen by Chinese astronomers who faithfully noted the event 

 in their records and stated that the new star was visible by day as 

 well as by night. According to modern terminology this was a 

 supernova. Research shows that about once every 500 years in our 

 galaxy a star reaches an unstable point in its evolution, whereupon 

 the whole star explodes like a giant atomic bomb. The disintegration 

 is complete and all that remains is an expanding ball of gas. Astrono- 

 mers have checked the rate of expansion spectroscopically and also 

 by taking photographs spaced many years apart. On extrapolating 



